The present invention relates generally to a method for removing acid mist from a gas stream. In particular, the invention relates to the purification of carbon dioxide by removing acid mist. The present invention has particular application in the purification of flue gas containing SOx and NOx generated by oxyfuel combustion of a fuel selected from hydrocarbon fuels and carbonaceous fuels, in which sulfur dioxide (SO2) and NOx are converted to at least sulfuric acid condensate and nitric acid condensate respectively.
The term “SOx” means oxides of sulfur and includes SO2 and sulfur trioxide (SO3). The term “NOx” means oxides of nitrogen and includes primarily nitric oxide (NO) and nitrogen dioxide (NO2). NOx may comprise one or more other oxides of nitrogen including N2O, N2O4 and N2O3.
It has been asserted that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The main greenhouse gas which is being emitted, carbon dioxide (CO2), has risen in concentration in the atmosphere from 270 ppm before the industrial revolution to the current figure of about 378 ppm. Further rises in CO2 concentration are inevitable until CO2 emissions are curbed. The main sources of CO2 emission are fossil fuel fired electric power stations and from petroleum fuelled vehicles.
The use of fossil fuels is necessary in order to continue to produce the quantities of electric power that nations require to sustain their economies and lifestyles. There is, therefore, a need to devise efficient means by which CO2 may be captured from power stations burning fossil fuel so that it can be stored rather than being vented into the atmosphere. Storage may be deep undersea; in a geological formation such as a saline aquifer; or a depleted oil or natural gas formation. Alternatively, the CO2 could be used for enhanced oil recovery (EOR).
The oxyfuel combustion process seeks to mitigate the harmful effects of CO2 emissions by producing a net combustion product gas consisting of CO2 and water vapor by combusting a carbonaceous or hydrocarbon fuel in pure oxygen. This process would result in an absence of nitrogen (N2) in the flue gas, together with a very high combustion temperature which would not be practical in a furnace or boiler. In order to moderate the combustion temperature, part of the total flue gas stream is typically recycled, usually after cooling, back to the burner.
An oxyfuel process for CO2 capture from a pulverized coal-fired power boiler is described in a paper entitled “Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver, Sep. 2004), the disclosure of which is incorporated herein by reference.
Oxyfuel combustion produces raw flue gas containing primarily CO2, together with contaminants such as water vapor; “non-condensable” gases, i.e. gases from chemical processes which are not easily condensed by cooling, such as excess combustion oxygen (O2), and/or O2, N2 and argon (Ar) derived from any air leakage into the system; and acid gases such as SO3, SO2, hydrogen chloride (HCl), NO and NO2 produced as oxidation products from components in the fuel or by combination of N2 and O2 at high temperature. The precise concentrations of the gaseous impurities present in the flue gas depend on factors such as on the fuel composition; the level of N2 in the combustor; the combustion temperature; and the design of the burner and furnace.
In general, the final, purified, CO2 product should ideally be produced as a high pressure fluid stream for delivery into a pipeline for transportation to storage or to site of use, e.g. in EOR. The CO2 must be dry to avoid corrosion of, for example, a carbon steel pipeline. The CO2 impurity levels must not jeopardize the integrity of the geological storage site, particularly if the CO2 is to be used for EOR, and the transportation and storage must not infringe international and national treaties and regulations governing the transport and disposal of gas streams.
It is, therefore, necessary to purify the raw flue gas from the boiler or furnace to remove water vapor; SOx; NOx; soluble gaseous impurities such as HCl; and “non-condensable” gases such as O2, N2 and Ar, in order to produce a final CO2 product which will be suitable for storage or use.
In general, the prior art in the area of CO2 capture using the oxyfuel process has up to now concentrated on removal of SOx and NOx upstream of the CO2 compression train in a CO2 recovery and purification system, using current state of the art technology. SOx and NOx removal is based on flue gas desulphurization (FGD) schemes such as scrubbing with limestone slurry followed by air oxidation producing gypsum, and NOx reduction using a variety of techniques such as low NOx burners, over firing or using reducing agents such as ammonia or urea at elevated temperature with or without catalysts. Conventional SOx/NOx removal using desulphurization and NOx reduction technologies is disclosed in “Oxyfuel Combustion For Coal-Fired Power Generation With CO2 Capture—Opportunities And Challenges” (Jordal et al; GHGT-7, Vancouver, 2004). Such process could be applied to conventional coal boilers.
US 2007/0122328 A1 (granted as U.S. Pat. No. 7,416,716 B1) discloses the first known method of removing SO2 and NOx from crude carbon dioxide gas produced by oxyfuel combustion of a hydrocarbon or carbonaceous fuel, in which the removal steps take place in the CO2 compression train of a CO2 recovery and purification system. This process is known as a “sour compression” process since acid gases are compressed with carbon dioxide flue gas. The method comprises maintaining the crude carbon dioxide gas at elevated pressure(s) in the presence of O2 and water and, when SO2 is to be removed, NOx, for a sufficient time to convert SO2 to sulfuric acid and/or NOx to nitric acid; and separating said sulfuric acid and/or nitric acid from the crude carbon dioxide gas.
It is well known that producing sulfuric acid and/or nitric acid using a condensation process can result in the formation of “acid mist” which is an aerosol of liquid acid condensate. In the context of the present invention, the term “aerosol” refers to a suspension of fine liquid droplets within a gas. The size of liquid droplets in an acid aerosol or mist such as a sulfuric acid mist may range from about 0.05 microns to about 10 microns (μm).
Acid mists must be removed efficiently from vent streams in order to meet stringent environmental emission regulations. In this connection, the maximum allowable emission into the atmosphere of sulfuric acid mist from sulfuric acid plants in the US is 0.15 lb (68 g) acid mist/ton 100% sulfuric acid produced, which works out to about 11 ppmv as an emission concentration. This maximum is similar to the European requirements. In addition, acid mists are highly corrosive and will damage downstream processing equipment if not removed to an adequate extent. Therefore, there is a need to reduce or eliminate acid mists in effluent gas from processes that form sulfuric acid and/or nitric acid condensates.
There is a problem, however, in that acid mists in general, and droplets of acid condensate having a mean average particle size of less than about 3 μm in particular, have proven to be very difficult to remove from gas streams.
Fiber bed mist eliminators have been used in the sulfuric acid industry to capture sulfuric acid mist prior to venting waste gas. Examples of the use of fiber bed mist eliminators in this way are disclosed in U.S. Pat. No. 3,540,190 A, U.S. Pat. No. 3,948,624 A, U.S. Pat. No. 4,348,373 A, U.S. Pat. No. 4,818,257 A and U.S. Pat. No. 5,139,544. In each case, the feed gas containing the acid mist is fed to the fiber bed mist eliminator(s) at about atmospheric pressure.
The Inventor is not aware of any industry that uses of fiber bed mist eliminators at elevated pressures.
U.S. Pat. No. 3,432,263 A discloses a process to produce sulfuric acid operating at an elevated pressure from about 5 bar to about 50 bar (0.5 MPa to 5 MPa) in which elemental sulfur is combusted in an oxyfuel combustion unit to produce SO2 which is in turn is oxidized catalytically to form SO3. The SO3 is then absorbed in water at a temperature from about 50° C. to about 200° C. to produce a concentrated aqueous sulfuric acid solution. Residual gas is heated, expanded and then vented to the atmosphere. The reference is, however, entirely silent on the issue of acid mist production and subsequent removal from the residual gas.
Since the amount of acid mist produced for a given volume of gas in flue gas desulfurization (FGD) processes is much less than in sulfuric acid processes, the focus to date in FGD processes has been primarily on preventing acid mists from forming in the first place rather than on removing acid mists that may have been formed. For example, U.S. Pat. No. 5,198,206 in the name of Haldor Topsøe A/S of Denmark discloses a process in which emission of sulfuric acid mist is maintained below 5 ppmv by “heterogeneous nucleation control” which involves the addition of a specific amount of smoke or aerosol of solid particles to a flow of gas containing SO3 and water upstream of a cooling tower or condenser. The solid particles may be produced by combusting hydrocarbons with more than 2 carbon atoms or silicones, or may be added as smoke from electric arc or welding. The solid particles act as minute nucleation cores that promote condensation of sulfuric acid. Indeed, according to promotional literature entitled “The Topsøe SNOX™ technology for cleaning flue gas from combustion of petroleum coke and high sulfur petroleum residues” (revised May 2006) produced by Haldor Topsøe A/S, formation of aerosol in the condenser is avoided by heterogeneous nucleation control. U.S. Pat. No. 4,348,373, also to Haldor Topsøe, discloses an alternative process for reducing acid mist formation that involves carefully controlling the temperature of an acid recycle stream in a sulfuric acid tower. Both of these processes operate at about atmospheric pressure.
The problem of sulfuric acid mist formation in FGD processes operating at about atmospheric pressure in power generation applications has also been addressed using wet electrostatic precipitators (WESPs) which are particulate collection devices that remove particles from a gas flow using an induced electrostatic charge. Processes involving the use of WESPs in this way are disclosed in JP 2009/195860 A and in “Experiences of wet type electrostatic precipitator successfully applied for SO3 mist removal in boilers using high sulfur content fuel” (Fujishima and Nagata; Mitsubishi Heavy Industries, Ltd.; ICESP IX conference; South Africa; May 17-21; 2004).
Mesh pads have been suggested for use to capture acid mists in an HCl scrubber operating at 122 psia (8.4 bar) and 82° F. (28° C.) (Case Study #1; “The Engineered Mist Eliminator” brochure; ACS Industries Inc., Houston, Tex. 77053, USA; 2004). However, whilst mesh pads are reasonably efficient at removing droplets in acid mists at elevated pressures having a mean diameter of more than about 5 μm, their use in elevated pressure applications is limited since they are not suited for removing droplets in acid mists having a mean diameter of less than about 5 μm. In this connection, mesh pads demonstrate good removal of about 99% of mist at a droplet size of 5 μm or above. However, the “d50” value for this type of mist eliminator is about 2.5 μm, i.e. only 50% removal is expected at the 2.5 μm droplet size which is where most of the acid mist droplet size is expected.
There is a need to develop new methods for the efficient removal of acid mists from streams of gas such as carbon dioxide at elevated pressure, particularly from crude carbon dioxide gas such as flue gas produced in an oxyfuel combustion process. The new methods should remove at least substantially all, and preferably all, of the acid mist. In addition, the new methods should be able to remove effectively droplets of an acid mist in an aerosol that have a mean diameter of less than about 5 μm. The methods should ideally also be less complicated and less expensive, both in terms of the capital and operating cost, than existing technologies.
In particular, it is an object of preferred embodiments of the present invention to improve the methods disclosed in US 2007/0122328 A1 by providing an effective means by which acid mist may be removed from the carbon dioxide gas at elevated pressure.